1. Field of the Invention
The present invention relates to a memory device. More particularly, the present invention relates to a static random access memory (SRAM) cell structure and manufacturing method thereof.
2. Description of the Related Art
As semiconductors having deep sub-micron features are fabricated, overall dimension of integrated circuit devices shrinks correspondingly. In the case of memory devices, overall dimension of each memory cell is also reduced. With the development of high tech electronic products (for example, computers, mobile phones, digital cameras or personal digital assistants), the amount of data that needs to be stored and process increases considerably. To meet the memory capacity demanded by these information technology products, smaller and higher quality integrated memory devices has to be developed.
Random access memory (RAM) is a type of volatile memory which has many applications in information technology products. In general, random access memory can be classified into static random access memory (SRAM) and dynamic random access memory (DRAM).
In each SRAM cell, digital data is stored as the conductive state of a transistor. Hence, a conventional SRAM cell consists of either a set of four transistors and two resistors (4T2R configuration) or a set of six transistors (6T configuration). On the other hand, digital data is stored as the charging state of a capacitor inside each DRAM cell. Accordingly, a conventional DRAM cell consists of a transistor and a capacitor (including a stacked capacitor or a deep trench capacitor.
In general, SRAM has a faster data processing speed and is possible to integrated with a complementary metal-oxide-semiconductor (CMOS) fabrication process. In other words, the process for manufacturing SRAM is simpler. However, one major drawback of incorporating SRAM is that area occupied by each cell is considerably larger (a SRAM cell with six transistors occupy an area roughly 10 to 16 times that of a DRAM cell). Yet, despite the considerable saving of area in incorporating DRAM cells, this advantage is counteracted by the complicated processes and hence the cost needed to manufacture the capacitor within each DRAM cell.
In recent years, a one transistor SRAM cell (1T-SRAM) (also referred to as a pseudo-SRAM) has been developed. The 1T-SRAM uses a DRAM memory cell (1T1C configuration) to replace the SRAM cell (6T or 4T2R configuration) but is able to maintain the peripheral circuit structure of the original SRAM. Therefore, memory cell dimension is reduced and the level of integration is increased, and yet the refresh-free property and small random access cycle time of a SRAM cell can be retained. In other words, the 1T-SRAM is a potential candidate for replacing the conventional SRAM and DRAM cells.
At present, most manufacturers use the DRAM manufacturing process (for example, in U.S. Pat. Nos. 6,468,855 and 6,573,548) to manufacture the IT-SRAM. Since the DRAM manufacturing process involve complicated steps, production cost is relatively high.